Generally, for industrial vehicles running under specific running conditions, such as operating on specific terrain (yard) or operating at night, maintenance of an appropriate running speed is usually required. Accordingly, it has been a common practice to equip the industrial vehicle with a running speed control apparatus for controlling the running speed of the industrial vehicle according to the running conditions or with an engine speed control apparatus for controlling the engine speed of the internal combustion engine and thereby limiting the maximum running speed of the industrial vehicle. A prior art running speed control apparatus is provided with an alarm means that generates an alarm signal to warn the driver upon detection of a running speed exceeding a predetermined maximum running speed. A prior art engine speed control apparatus is provided with a speed reducing means that automatically reduces the engine speed to idling speed upon detection of a running speed exceeding a predetermined maximum running speed.
FIG. 22 is a diagrammatic illustration of an example of the above-mentioned prior art, i.e., an apparatus for controlling the running speed of an industrial vehicle through the control of the speed of the internal combustion engine of an the industrial vehicle.
Referring to FIG. 22, when the accelerator pedal 2 of a vehicle mounted with an engine 1 is depressed, a throttle lever 4 connected to the accelerator pedal 2 by a throttle cable 3 opens the throttle valve 5 of a carburetor 6, potentially to its fully open position, and when the displacement of the accelerator pedal is reduced, the opening of the throttle valve 5 is reduced accordingly. Thus, the speed of the engine is controlled by operating the accelerator pedal 2.
An actuator 7, such as an electromagnetic clutch having a pair of clutch plates, is provided on a transmission line for transmitting the movement of the accelerator pedal to the throttle lever 4. Normally, the actuator 7 is in an off-state (for the electromagnetic clutch, a state such that the pair of clutch plates are engaged) and the throttle valve 5 is operated according to the displacement of the accelerator pedal 2. A running speed sensor 8 provides a running speed signal representing the running speed of the vehicle to an ECU (electronic control unit) 10 comprising a microcomputer. Upon detection of a running speed exceeding an upper limit running speed from the running speed signal provided by the running speed sensor 8, the ECU 10 turns on the actuator 7 (for the electromagnetic clutch, a state such that the pair of clutch plates are disengaged). With the actuator 7 turned on, the depression displacement of the accelerator pedal 2 is not transmitted to the throttle cable 3, so that the throttle valve 5 returns to its fully closed position and the engine 1 operates at idling speed.
When the accelerator pedal 2 is released, the accelerator pedal 2 closes an idling switch 9, and upon receipt of a signal through the idling switch 9, the ECU 10 generates a signal to turn off the actuator 7. Consequently, control of the throttle valve 5 using the accelerator pedal 2 is resumed.
Although the prior art running speed control system provided with the alarm means generates an alarm signal, the running speed of the vehicle cannot be actually controlled unless a driver carries out a necessary decelerating operation including releasing the accelerator pedal and applying the brake.
When the engine is controlled by the prior art engine speed control apparatus capable of automatically reducing the engine speed to idling speed, a driver will experience unpleasant deceleration because the engine speed drops sharply from a high speed to idling speed. Furthermore, it is inconvenient for the accelerator pedal to be released in order to restore the normal function of the accelerator pedal.
On the other hand, the load of the hydraulic pump for operating the cargo handling system, the power steering system and the brake mechanism, in addition to the running load, acts on the internal-combustion engine of the industrial vehicle. The internal-combustion engine provides output torque necessary for maintaining the idling speed during an idling operation. Therefore, the engine speed will drop below the idling speed and, in some cases, the internal-combustion engine will stall if an excessively large load exceeding the torque acts on the internal-combustion engine during an idling operation. The power steering system of a forklift truck, in particular, is frequently operated during an idling operation, and the internal-combustion engine often stalls when the steering mechanism is turned through a large angle during the idling operation, because the torque required for driving the hydraulic pump to steer the forklift truck when the steering mechanism is turned through a large angle exceeds the idling torque of the internal-combustion engine. Accordingly, an idling speed increasing device as shown in FIG. 23 has been employed to overcome such a disadvantage.
A throttle valve 5 is pivotally supported by a throttle shaft 5a within the barrel of a carburetor 6. A U-shaped suction pipe 11 has one end connected to the carburetor 6 at a position on the suction pipe side of the engine with respect to the throttle valve 5, and the other end is connected to a throttle valve operating device 12.
The throttle valve operating device 12 comprises a diaphragm case 13, a diaphragm 14 partitioning the interior of the diaphragm case 13 into two chambers, and a throttle valve operating rod 15 attached to the diaphragm 14 for operating the throttle valve 5. A spherical throttle valve pushing member 15a is attached to the free end of the throttle valve operating rod 15. The negative pressure or vacuum prevailing within the suction pipe 11, and the front chamber 16 of the diaphragm case 13 communicating with the suction pipe 11, is equal to the intake pressure, i.e., a negative pressure, prevailing within the carburetor 6. The diaphragm 14 retains a normal shape while the pressure in the front chamber 16 is equal to the ambient pressure, i.e., the atmospheric pressure acting on the idling speed increasing device 12. The diaphragm 14 bulges into the front chamber 16 when the pressure in the front chamber 16 decreases below the atmospheric pressure. Thus, the diaphragm 14 retains a normal shape when the intake pressure prevailing within the carburetor 6 is equal to the atmospheric pressure and bulges into the front chamber 16 when the intake pressure decreases below the atmospheric pressure.
When the diaphragm 14 bulges into the front chamber 16, the throttle valve operating rod 15 is retracted accordingly into the diaphragm case 13 and, as the degree of projection of the diaphragm 14 decreases, the throttle valve operating rod 15 advances accordingly into the carburetor 6. Thus, the throttle valve operating rod 15 is retracted toward the front chamber 16 when the intake pressure prevailing within the carburetor 6 decreases below atmospheric pressure and advances into the carburetor 6 as the intake pressure approaches atmospheric pressure.
Incidentally, the throttle valve 5 is held at a predetermined idling opening sufficiently large to maintain the idling speed during an idling operation. Therefore, the engine speed drops below idling speed if the engine is loaded with an additional load during the idling operation. Under such operating conditions, the idling speed can be recovered by increasing the air intake rate, that is, the engine speed can be increased by increasing the opening of the throttle valve 5 according to the air demand.
The intake pressure increases when the engine speed decreases below idling speed due to the effect of an additional load acting on the engine during an idling operation, and the degree of projection of the diaphragm 14 into the front chamber 16 then decreases accordingly, and the throttle valve operating rod 15 advances into the carburetor 6. Consequently, the throttle valve pushing member 15a pushes the throttle valve 5 to open the same, and the opening of the throttle valve 5 increases to increase the air intake rate as the intake vacuum decreases.
When the engine speed decreases below idling speed, the idling speed increasing device shown in FIG. 23 increases the opening of the throttle valve 5 according to a reduction of the engine speed thereby increasing the engine speed by increasing the air intake rate. Accordingly, the engine will not stall even if the engine is loaded with an excessively large load during an idling operation.
Another method of preventing an engine from stalling when subjected to an excessively large load during an idling operation employs an electronic governor that controls the engine speed in a PID control (proportional-plus-integral-plus-derivative control) instead of the foregoing engine speed increasing device.
This electronic governor detects actual engine speed from an ignition signal provided by the ignition system, determines the deviation of the detected actual engine speed from a desired engine speed, i.e., the idling speed, and carries out a PID operation for PID control at a gain determined from the deviation to determine a desired opening of the throttle valve 5 necessary for maintaining the idling speed so as to prevent the engine from stalling.
However, the engine speed increasing device is unable to regulate and change the opening of the throttle valve 5 optionally according to engine operating conditions when the engine speed decreases, and the intake pressure does not increase instantaneously in response to the drop in engine speed; the intake pressure starts increasing with a time lag after a reduction in engine speed. Therefore, the engine speed increasing device is unable to increase the opening of the throttle valve 5 instantly upon application of an additional load to the engine and is unable to cope with a sharp engine load variation. Thus, the engine speed increasing device has a problem in its response characteristics.
When power is required to drive the hydraulic pump for operating the steering system while the engine is idling, the variation time of the pressure of the working fluid discharged from the hydraulic pump to operate the steering system, which will be referred to as "power-steering pressure 17", corresponds to the variation time of the power for driving the hydraulic pump as shown in FIG. 24. When the engine speed is controlled by the foregoing electronic governor, the period of the ignition signal 18 increases with an increase in the power steering pressure 17 and, consequently, the engine speed decreases. However, the increase in the period of the ignition signal 18 does not respond instantly to an increase in the power-steering pressure 17, that is, the reduction in engine speed lags behind the increase of the load on the engine. As shown in FIG. 24, the engine stalls at time A.
The intake pressure 19 of the carburetor increases as the power-steering pressure 17 increases. However, a variation of the intake pressure 19 does not respond instantly to a variation of the cyclic period of the ignition signal 18; that is, the intake pressure 19 increases with a time lag after a reduction in engine speed.
In a four-cycle four-cylinder engine, ignition occurs twice for each revolution of the output shaft. Therefore, the electronic governor determines the actual engine speed from the sum of the cyclic periods of the two ignition signals 18 each time the electronic governor receives two ignition signals 18, and carries out the PID control operation with reference to actual engine speed. Therefore, if the load on the engine changes suddenly while the electronic governor receives the two ignition signals 18, the electronic governor is unable to detect the sudden variation of the load on the engine. Thus, the electronic governor has a problem in its response characteristics.
If the electronic governor is capable of determining a desired throttle valve opening by determining actual engine speed each time the ignition signal 18 is given to the electronic governor, the electronic governor may be able to deal with a sudden load change during an idling operation and the response characteristics of the electronic governor may be improved. However, if the opening of the throttle valve is controlled in such a mode, the opening of the throttle valve will change in response to a slight change in the period of the ignition signal 18 when the engine operates at a high engine speed, which makes the engine speed unstable.
The cyclic period of the ignition signal 18 at time B when the power-steering pressure 17 is increased is longer than the cyclic period of the ignition signal 18 at time C when the engine is idling and the power-steering pressure 17 is not increased. The engine can be prevented from stalling if the opening 20 of the throttle valve is increased at time B to increase the air intake rate; that is, stalling can be avoided by determining the deviation of the actual engine speed at time B from the idling speed and determining the gain for PID control from the deviation. However, the gain thus determined is excessively large for the PID control of engine speed when the engine operates at a high speed, which also makes the engine speed unstable.
In FIG. 24, the opening 20 of the throttle valve remains unchanged regardless of an increase in the power-steering pressure 17, because the gain for the PID control of the engine speed is determined so that the gain is not excessively large for controlling the engine speed while the engine is operating at a high speed.
Thus, the ability of the electronic governor to adjust and change the opening of the throttle valve for the actual engine speed optionally by changing the gain of the PID control is superior to the ability of the idling speed increasing device shown in FIG. 23. However, the electronic governor has contradictory problems in that the response characteristics of the electronic governor are not satisfactory during an idling operation if the engine speed control mode is determined primarily for a high engine speed and the stability of the engine speed deteriorates when the engine operates at a high speed if the engine speed control mode is determined primarily for the idling speed.